If you are asking how to turn your GS3 into a global phone so that you can travel overseas and put in a foreign SIM, then the official answer is "sometime in the future". Verizon has promissed a Global update. However you will never be able to use a T-Mobile or AT&T SIM here in the USA. If you are asking for TOS breaking advice, then there are other forums for that kind of question.

If neither of these are even close to what you are asking, please provide more details.

In "Settings".."Security" there is an option to "Set Up Sim Card Lock". There you can set a pin to lock the card. As I have not used it I don't know if there is also an option there to UNlock it, after you have locked it.